WP Super Cache generates static HTML files from your dynamic WordPress blog. After an HTML file is generated, your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts.
Serving static HTML files
The static HTML files will be served to the vast majority of your users:
- Users who are not logged in.
- Users who have not left a comment on your blog.
- Or users who have not viewed a password-protected post.
99% of your visitors will be served static HTML files. One cached file can be served thousands of times. Other visitors will be served custom cached files tailored to their visit. If they are logged in or have left comments, those details will be displayed and cached for them.
Comments will show as soon as you moderate them, depending on the blog’s comment policy. Other dynamic elements on a page may not update unless they are written in Javascript, Java, or another client-side browser language. The plugin really produces static html pages. No PHP is executed when those pages are served.
How WP Super Cache Works
The plugin serves cached files in 3 ways (ranked by speed):
- Expert. The fastest method is using Apache mod_rewrite (or whatever similar module your web server supports) to serve “supercached” static HTML files. This completely bypasses PHP and is extremely quick. If your server is hit by a deluge of traffic, it is more likely to cope as the requests are “lighter”. This does require the Apache mod_rewrite module (which is probably installed if you have custom permalinks) and a modification of your
.htaccess file, which is risky and may take down your site if modified incorrectly.
- Simple. Supercached static files can be served by PHP, the recommended way of using the plugin. The plugin will serve a “supercached” file if it exists, and it’s almost as fast as the mod_rewrite method. It’s easier to configure as the
.htaccess file doesn’t need to be changed. You still need a custom permalink. You can keep portions of your page dynamic in this caching mode.
- WP-Cache caching. This is mainly used to cache pages for known users, URLs with parameters, and feeds. Known users are logged-in users, visitors who leave comments, or those who should be shown custom per-user data. It’s the most flexible caching method and slightly slower. WP-Cache caching will also cache visits by unknown users if supercaching is disabled. You can have dynamic parts to your page in this mode too. This mode is always enabled, but you can disable caching for known users, URLs with parameters, or feeds separately. Set the constant
DISABLE_SUPERCACHE to 1 in your wp-config.php if you want only to use WP-Cache caching.
Super Cache files are compressed and stored that way so the heavy compression is done only once. These files are generally much smaller and are sent to a visitor’s browser much more quickly than uncompressed HTML. As a result, your server spends less time talking over the network which saves CPU time and bandwidth, and can also serve the next request much more quickly.
If you’re uncomfortable editing PHP files, use simple mode. It’s easy to set up and very fast.
Installing WP Super Cache
The WP Super Cache plugin can be installed from your site’s WP Admin dashboard. To install the WP Super Cache plugin via WP Admin:
- Go to Plugins → Add New.
- Search for WP Super Cache. The latest version will show in the search results.
- Click Install Now.
- Go to the plugin settings page at Settings -> WP Super Cache and enable caching.
Recommended Settings for WP Super Cache
- Simple caching.
- Compress pages.
- Don’t cache pages for known users.
- Cache rebuild.
- CDN support.
- Extra homepage checks.
Garbage collection is the act of cleaning up cache files that are out of date and stale. There’s no correct value for the expiry time, but a good starting point is 1800 seconds.
Consider deleting the contents of the “Rejected User Agents” text box and allow search engines to cache files for you.
Preload as many posts as you can and enable “Preload Mode”. Garbage collection of old cached files will be disabled. If you don’t care about sidebar widgets updating often set the preload interval to 2880 minutes (2 days) so all your posts aren’t recached very often. When the preload occurs, the cache files for the post being refreshed is deleted and then regenerated. Afterward, a garbage collection of all old files is performed to clean out stale cache files. Even with preload mode enabled, cached files will still be deleted when posts are modified or comments made.
Additional Settings for WP Super Cache
Below are the main settings offered in WP Super Cache. If you need more information, you can look at the Developer documentation.
Preloading
You can generate cached files for your site’s posts, categories, and tags by preloading. Preloading will visit each page of your site, generating a cached page as it goes along, just like any other visitor to the site. Due to the sequential nature of this function, it can take some time to preload a complete site if there are many posts.
To make preloading more effective, it can be useful to disable garbage collection so that older cache files are not deleted. This is done by enabling “Preload Mode” in the settings. Be aware that pages will go out of date eventually but that updates by submitting comments or editing posts will clear portions of the cache.
Garbage Collection
Your cache directory fills up over time, which takes up space on your server. If space is limited or billed by capacity, or if you worry that the cached pages of your site will go stale, then garbage collection has to be done. Garbage collection happens regularly and deletes old files in the cache directory. On the advanced settings page, you can specify the following:
- Cache timeout. How long are cache files considered fresh for. After this time, they are stale and can be deleted.
- Scheduler. Set up how often garbage collection should be done.
- Notification emails. You can be informed on garbage collection job progress. There are no right or wrong settings for garbage collection. It depends on your site. If your site gets regular updates or comments, set the timeout to 1800 seconds and the timer to 600 seconds. If your site is mostly static, you can disable garbage collection by entering 0 as the timeout or use a really large timeout value.
The cache directory, usually wp-content/cache/ is only for temporary files. Do not ever put important files or symlinks to important files or directories in that directory. They will be deleted if the plugin has write access to them.
Using a CDN with WP Super Cache
A Content Delivery Network (CDN) is usually a network of computers situated around the world that will serve the content of your website faster by using servers close to you. Static files like images, Javascript, and CSS files can be served through these networks to speed up how fast your site loads. You can also create a “poor man’s CDN” by using a subdomain of your domain to serve static files too.
OSSDL CDN off-linker has been integrated into WP Super Cache to provide basic CDN support. It works by rewriting the URLs of files (excluding .php files) in wp-content and wp-includes on your server so they point at a different hostname. Many CDNs support origin pull. This means the CDN will download the file automatically from your server when it’s first requested, and will continue to serve it for a configurable length of time before downloading it again from your server.
Configure this on the “CDN” tab of the plugin settings page. This advanced technique requires a basic understanding of how your webserver or CDNs work. Please be sure to clear the file cache after you configure the CDN.
REST API
There are now REST API endpoints for accessing the settings of this plugin. You’ll need to be authenticated as an admin user with permission to view the settings page to use it. This has not been documented yet, but you can find all the code that deals with this in the “rest” directory.
Custom Caching in WP Super Cache
It is now possible to hook into the caching process using the add_cacheaction() function.
Three hooks are available:
wp_cache_get_cookies_values – modify the key used by WP Cache.add_cacheaction – runs in phase2. Allows a plugin to add WordPress hooks.cache_admin_page – runs on the admin page. Use it to modify that page, perhaps by adding new configuration options.
There is one regular WordPress filter too. Use the “do_createsupercache” filter to customize the checks made before caching. The filter accepts one parameter. The output of WP-Cache’s wp_cache_get_cookies_values() function.
WP Super Cache has its own plugin system. This code is loaded when WP Super Cache loads and can be used to change how caching is done. This is before most of WordPress loads, so some functionality will not be available. Plugins can be located anywhere that PHP can load them. Add your own plugin either:
- by putting your plugin in the
wp-content/plugins/wp-super-cache-plugins directory, or
- by calling
wpsc_add_plugin( $name ) where $name is the full filename and path to the plugin. You only need to call that function once to add it. Use wpsc_delete_plugin( $name ) to remove it from the list of loaded plugins.
The cookies WP Super Cache uses to identify “known users” can be modified now by adding the names of those cookies to a list in the plugin configuration. Use wpsc_add_cookie( $name ) to add a new cookie, and wpsc_delete_cookie( $name ) to remove it. The cookie names also modify the mod_rewrite rules used by the plugin, but I recommend using Simple mode caching to avoid complications with updating the .htaccess file. The cookie name and value are used to differentiate users so that you can have one cookie but different values for each type of user on your site, for example. They’ll be served different cache files.
